Tin and tin alloy plating bath



Ma/ch 31, 1953 J. w. ANDREWS 2,633,450

TIN AND TIN ALLOY PLATIN@ BATH Filed May 27, 1949 FIE- .ma im FIELE'.-

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Patented Mar. 31, 1953 TIN AND TIN ALLOY PLAT-ING BATH John W. Andrews, Pittsburgh, Pa., assignor to United States Steel Corporation, a corporation of New Jersey Application May 27, 1949, Serial No. 95,831

This invention relates to an electrolyte composition for the electrodeposition of tin and tin alloys whereby dense, smooth, highly ductile coatings of good luster can be deposited over a wide range of cathode current densities. The invention relate-s particularly to an improved addition agent for use in acid tin and tin alloy plating baths.

Economic considerations demand that the equipmentfor electrotinning continuous strip be run at the highest possible speed and be designed to occupy a minimum of floor space. The top limit of speed and the space requirements are largely iixed by the maximum current density at which the plating bath will produce acceptable coatings. The size and cost of slack producers and similar auxiliaries are fixed largely by the minimum current density at which the bath will produce acceptable deposits. Consequently, since the introduction of continuous strip plating there has been a continued and intensive effort to improve these operating characteristics of plating solutions.

Acid type baths are preferred for tinplating for reasons of economy. In such baths the maximum and minimum current densities are known to be particularly affected by the presence in the bath yof small amounts of substances termed addition agents. Tin plating baths composed of various acids and salts and containing one or more additionagents such as glue, acetone, cresol, aloes, and complex sulphonated cresol, tars, and oils have been tried. Some of these baths possess fairly wide operating current density ranges when used on a small scale. Any deiciencies of a plating bath, however, are empha- Sized in strip plating because of the necessarily large volume of solution used, the high operating speeds, and the continuous nature of the operation. None of the baths of the prior art have proved entirely satisfactory in commercial electrotinning of steel strip. The troubles are due mainly to instability and/or variations in activity of the addition agents which result in uncontrollable shifts in the operating characteristics of the bath and in turn in large quantities oi oli-grade plate.

I have overcome the foregoing deficiencies lof acid tin and tin alloy plating baths by using as an addition agent a mixture of the 4-4 and 2-4 isomers of dihydroxy diphenyl sulphone.

Iam aware that Stack in U. S. Patent No. 2,313,371, granted March 9, 1943, teaches the use of certain aromatic sulphones as addition agents in acid type tin baths, particularly in baths of the phenolsulphonic acid type. I n commercial 8 Claims. (Cl. 204-54) operations, however, it was found that while Stacks agents gave far better results than any previously proposed, the activity of his sulphone agents varied Widely and in many instances the expected current density range was not attained, thus resulting in serious production losses, Stacks teachings suggest `that impurities in the agents are responsible for the variations. While variations in activity are considerably reducedby using very pure material, this is accomplished only at the expense of reducing the operating current density range.

Accordingly, it is an object of the present invention to provide an electrolyte composition for the electrodeposition oi tin vand tin alloys whereby uniform, dense, adherent coatings of good lustei` can be reliably obtained over a wide range .of current densities.

` Another object of the invention is to provide an improved addition agent for use in acid tin and tin-alloy plating baths which will permit operation of the baths at high current density and over a wide range of current densities.

Still another object is to provide an improved tin plating bath for use in the continuous electrotinning of steel strip whereby higher strip speeds may be attained and the size and cost of equipment reduced.

These and other objects will become more ap parent from the following description when read in conjunction with the attached drawing wherein; i

Figure 1 is a graph showing the elect of the addition of small quantities of certain mixtures of the 4-4 and the 2-4' isomers of dihydroxy diphenyl sulphone upon the operating. current density range of three varieties of acid tin platine baths; and

Figure 2 is a graph showing the optimum concentrations of various mixtures of 4-4 and 2-4 dihydroxy diphenyl sulphone in three varieties of acid tin plating baths.

Referring specifically to Figure l, the upper curves l, 2 and 3 define the maximum current densities, the lower curves I', 2 and 3', the rmin-l imum current densities at which acceptable coatings are obtained; the operating range being vthe values of current density between any two curves of like number. Acceptable coatings are deiined as coatings which will melt to produce bright smooth tin surfaces free of dull streaks vand/or coalesced areas. The latter are indicative of burnt and feathery deposits which are associated with improper plating conditions. Such areas cannot always be detected lonthe plate in the ing: Tin g1/l-- 35 Sulphuric acid g./l 49 Curves 2-2 of Figure l and curve 2 of Figure 2 apply to a solution containing :1 s

Tin -g./l Sulphuric acid g./l Phenolsulphonic acid g./l 32 Curves 3-3 of Figure l and curve 3Y of Figure 2 apply to a solution containing:

'Iin g./l 35 Sulphuric acid g./l 33 Phenolsulphonic acid g./1 57 The data pertain speciiically to plating operations conducted at a temperature of 100 F. and a degree of circulation of the plating solution with respect to the face of the cathode of 600 F. P. M. In all solutions the free acidity is the equivalent of 20 grams per liter of sulphuric acid, and the concentration of addition agent is 1 grain per liter less than its maximum solubility in the bath.

Figure 1 illustrates my discovery, which has now been proved by extensive tests under commercial conditions, that not only is purity of the agent essential but that the agent must consist of a mixture of the 4-4 and 2-4 isomers of dhydroxy diphenyl sulphone in certain critical proportions. An addition agent compounded as I will describe, when used in aqueous acid tin plat- Hydmuobonc acid m 45 85 ing baths, particularly in baths based on sulphuric acid, phenolsulphonic acid, a mixture of phenolsulphonc and sulphuric acids, and hydrouoboric acid, will produce smooth, dense and highly ductile tin depositions on steel strip over a particularly wide range of current densities. The new agent isa mixture containing to 85% of 4-4' dihydroxy diphenyl sulphone and 40 to to 15% of 2-4' dihydroxy diphenyl sulphone. The

mixture must be substantially free, i. e. to contain less than 1%, of tarry matter which is normally present as the result of by-products of the sulfonation reaction in the preparation of materials. 'Ihe tarry matter is especially harmful as it is mechanically occluded in the coating and interferes with solderability and imparts undesirable taste to certain food products.

The amount of' the new agent which must be used may be broadly defined as between 50 and 100% of its maximum solubility in the plating bath. The upper limit of the current density range is attained at approximately of the solubility limit. The lower limit of the range is not attained until the concentration of the agent has been increased to within approximately one gram per liter of its solubility limit in the electrolyte. The solubility Varies somewhat with the type of acid, the free acidity of the solution, the amount of tin present and the temperature at which the bath is operated. In practice, therefore, it is preferable to use not less than two grams per liter and not more than approximately one gram per liter less than the quantity required to saturate the bath with the agent. This practice is desirable since in a continuous operation the acid content and tempera-ture of the solution cannot be maintained absolutely constant. By holding concentration of the agent within the preferred limits, precipitation of the agent in the heat exchangers which must be used to maintain the solution Within its operative temperature range will be avoided. This has been one of the major sources of trouble when using the plating baths of the prior art in a continuous strip tinning operation.

The preferred upper concentration limit of the new agent in three varieties of baths is shown in Figure 2. vIt will be noted that in solutions containing both sulphuric and phenolsulphonic acid the solubility of the agent increases as the weight ratio of phenolsulphonic to sulphuric acid increases. I have found that when using an electrolyte containing both of these acids as the solvent for the tin the weight ratio of phenolsulphonic acid to sulphuric should be maintained between 1:1 and 2:1.

The new agents can be used in a variety of acid baths. However I prefer baths containing sulphuric acid, phenolsulphonic acid, a mixture of phenolsulphonic and sulphuric acids, or hydrouoboric acid. The bath compositions referred to in Figures 1 and 2 are very satisfactory. Plating baths composed as follows are particularly ei'lcient:

Grams per liter Addition agent 6-12 Equally good results are obtained with fluoboric acid type electrolytes of the following composition:

Grams per liter Tin 20-40 Addition agent 1-4 Sulphate baths of the following composition are also very eicient:

In the foregoing examples the term addition agent is limited to my mixture of 4-4 and 2-4 isomers of dihydroxy diphenyl sulphone which consists of 60 to 85% by weight of the 4-4' isomer and the balance essentially the 2-4( isomer. The baths are preferably operated at a temperature of to 120 F. They afford an operating current density range of 25-600 amp. per square foot.

The use of the mixture of the two isomers of. dihydroxy diphenyl sulphone in accordance with my invention provides an improved acid electrolyte which is particularly adapted to the con-V tinuous electrotinning of strip steel.V Uniform, lustrous, adherent tin coatings are reliably produced at current densities from 25-600 amp. per square foot. This increase in the current density range and the reliability of results permits coating at higher speeds without increasing the bath length and at the same time the yield of prime product is increased.

My agent can also be used to provide improved baths for the continuous plating of tin alloys, particularly lead-tin alloy coatings known to thetrade as terne coatings. In plating coatings con;y

taining lead, however, the electrolyte must not contain sulphate since the same would precipitate the lead.

An electrolyte which is particularly satisfactory for plating terne coatings is composed as follows:

Grams per liter Phenolsulphonic acid 65-330 Tin and lead -80 Addition agent 5-25 The ratio of tin to lead in the solution is adjusted to give the desired alloy composition of the coating; the ratio of metals in the bath being substantially the same as their ratio in the resulting coating.

This application is a continuation-in-part of my application Serial No. 90,007, led April 27, 1949, and `abandoned June 14, 1949.

While I have shown and described certain preferred embodiments of the invention, it is apparent that other adaptations and modications may be made without departing from the scope of the following claims.

I claim:

l. A bath for the electrodeposition of tin comprising an aqueous acid solution or tin, said acid being selected from the group consisting of hydrofluoboric acid, phenolsulphonic acid, sulphuric acid, and mixtures of the last two named acids, and, as an addition agent, an eiiective amount of a mixture consisting of 60 to 85% by weight 4-4 dihydroxy diphenyl sulphone and 49 to 15% 2-4 dinydroxy diphenyl sulphone; said effective amount being between 50 and 106% of said mixtures maximum solubility in said bath.

2. A bath for the electrodeposition of tin comprising an aqueous acid solution of tin, said acid being selected from the group consisting oi hydroiluoboric acid, phenoisulphonic acid, sulphuric acid, and mixtures of the last two named acids, and, as an addition agent, an eiective amount of a mixture consisting of 6G to 85% by weight 4-4 dihydroxy diphenyl sulphone and 40 to 15% 2-4 dihydroxy diphenyl sulphone; said elective amount being at least 2 grams per liter and not more than l gram per liter less than the maximum solubility of said mixture in said bath.

3. A bath for the electrodeposition of tin comprising an aqueous solution of 3G to 60 grams per liter of phenolsulphonic acid, to 4.5 grams per liter of sulphuric acid, 30 to 50 grams per liter of tin and 5 to l2 grams per liter of an addition agent; said addition agent being a mixture consisting of 60 to 85% by weight of 4-4 dihydroxy diphenyl sulphone, and the balance essentially 2-4' dihydroxy diphenyl sulphone.

4. A bath for the electrodeposition of tin comprising an aqueous solution of to 85 grams per liter of hydroiiuoboric acid, 20 to 40 grams per liter of tin and 1 to 4 grams per liter of an addition agent; said addition agent being a mixture consisting of 6i) to 85% by weight of 4-4 dihydroxy diphenyl sulphone and the balance essentially 2-4 dihydroxy diphenyl sulphone.

5. A bath for the electrodeposition of tin comprising an aqueous solution of l5 to 50 grams per liter of sulphuric acid, 3() to 50 grams per liter of tin and 2 to 5 grams per liter of an addition agent; said addition agent being a mixture consisting of 60 to 85% by weight of 4-4 dihydroxy diphenyl sulphone and the balance essentially 2-4 dihydroxy diphenyl sulphone.

6. In an aqueous acid bath for the electrodeposition of a metal coating of the class consisting of tin and tin alloys, an addition agent consisting of 60 to 85% by weight of 4-4 dihydroxy diphenyl sulphone and the balance essentially 2-4' dihydroxy diphenyl sulphone, said agent being present in an amount between and 100% of the maximum solubility of said agent in said bath.

7. In an aqueous acid bath for the electrodeposition of a metal coating of the class oonsisting of tin and tin alloys, an addition agent consisting of to 85% by Weight of 4-4 dihydroxy diphenyl sulphone and the balance essentially 2-4 dihydroxy diphenyl sulphone, said agent being present in an amount not less than 2 grams per liter and not more than 1 gram per liter less than the maximum solubility of said agent in said bath.

8. A bath for the electrodeposition of tin-lead ailoys comprising an aqueous solution of to 330 grams per liter of phenol sulphonic acid, 20 to 80 grams per liter of tin and lead, the ratio of tin to lead within said range adjusted to give the desired alloy composition and 5 to 25 grams per liter of an addition agent, said agent being a mixture consisting of 6G to 85 percent by Weight of 4-4' dihydroxy diphenyl sulphone and the balance essentially 2-4i dihydroxy diphenyl sulphone.

JOHN W. ANDREWS.

.REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,238,282 Huismann June 30, 1942 2,313,371 Stack Mar. 9, 1943 2,392,137 Foster Jan. l, 1946 2,399,194 Andrews Apr. 30, 1946 OTHER REFERENCES Journal of the American Chemical Society, vol. 67 (1945 ,Dl0- 23d-240. 

1. A BATH FOR THE ELECTRODEPOSITION OF TIN COMPRISING AN AQUEOUS ACID SOLUTION OF TIN, SAID ACID BEING SELECTED FROM THE GROUP CONSISTING OF HYDROFLUOBORIC ACID, PHENOLSULPHONIC ACID, SULPHURIC ACID, AND MIXTURES OF THE LAST TWO NAMED ACIDS, AND, AS AN ADDITION AGENT, AN EFFECTIVE AMOUNT OF A MIXTURE CONSISTING OF 60 TO 85% BY WEIGHT 4-4'' DIHYDROXY DIPHENYL SULPHONE AND 40 TO 15% 2-4'' DIHYDROXY DIPHENYL SULPHONE; SAID EFFECTIVE AMOUNT BEING BETWEEN 50 AND 100% OF SAID MIXTURE''S MAXIMUM SOLUBILITY IN SAID BATH. 